Why doesn't PIVOT clause work with COLLECT aggregate function in 11g ?

Question

Hello all !
I am really puzzled as to what is considered an aggregate function in the context of the PIVOT clause in 11g.
I have been toying with quite a few things related to collections lately and this arose as an aside :
CREATE TABLE TEST_COLL
   NODE_ID    VARCHAR2(15 CHAR) NOT NULL,
   NODE_VALUE VARCHAR2(45 CHAR) NOT NULL,
   NODE_LEVEL NUMBER(1)         NOT NULL
CREATE OR REPLACE TYPE TREE_NODE AS OBJECT
   NODE_KEY  VARCHAR2( 15 CHAR),
   NODE_NAME VARCHAR2(127 CHAR)
CREATE OR REPLACE TYPE TREE_NODES AS TABLE OF TREE_NODE NOT NULL;At this stage I am sure we all agree that the query
SELECT NODE_LEVEL,
       CAST(COLLECT(TREE_NODE(NODE_ID, NODE_VALUE)) AS TREE_NODES) AS NODES
  FROM TEST_COLL
GROUP BY NODE_LEVEL;is perfectly valid as the COLLECT function is an aggregate function according to the [Official Documentation|http://docs.oracle.com/cd/E11882_01/server.112/e10592/functions031.htm#i1271564]
But then, one of the following two queries should work
SELECT CAST(REGION_NODES     AS TREE_NODES) AS REGIONS,
       CAST(DEPARTMENT_NODES AS TREE_NODES) AS DEPARTMENTS,
       CAST(AREA_NODES       AS TREE_NODES) AS AREAS,
       CAST(CENTRE_NODES     AS TREE_NODES) AS CENTRES
  FROM (SELECT NODE_LEVEL, TREE_NODE(NODE_ID, NODE_VALUE) AS NODE
          FROM TREE_COLL
PIVOT (COLLECT(NODE) FOR NODE_LEVEL IN (1 AS REGION_NODES,
                                         2 AS DEPARTMENT_NODES,
                                         3 AS AREA_NODES,
                                         4 AS CENTRE_NODES
or (better)
SELECT REGION_NODES     AS REGIONS,
       DEPARTMENT_NODES AS DEPARTMENTS,
       AREA_NODES       AS AREAS,
       CENTRE_NODES     AS CENTRES
  FROM (SELECT NODE_LEVEL, TREE_NODE(NODE_ID, NODE_VALUE) AS NODE
          FROM TREE_COLL
PIVOT (CAST(COLLECT(NODE) AS TREE_NODES) FOR NODE_LEVEL IN (1 AS REGION_NODES,
                                                             2 AS DEPARTMENT_NODES,
                                                             3 AS AREA_NODES,
                                                             4 AS CENTRE_NODES
       );yet, both fail with
ORA-56902: expect aggregate function inside pivot operationInvestigating further, I found the same behaviour when using XMLAGG as the aggregate function in the PIVOT clause.
Is this normal ? And if it is, is there any other way to achieve the result I was anticipating ?
My version is
SQL> SELECT BANNER FROM V$VERSION;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE    11.2.0.3.0      Production
TNS for 64-bit Windows: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - ProductionThanks in advance
Best Regards
Philip

Answer 1

Most likely a bug. But you can bypass it by using any other aggregate making sure group consists of a single row and apply collect to a pivoted value. Yes, the cost is double aggregation. And also there is another cost - you would need to create MAP member function otherwise aggreations like MAX/MIN, etc. will not work:
CREATE OR REPLACE TYPE TREE_NODE AS OBJECT
   NODE_KEY  VARCHAR2( 15 CHAR),
   NODE_NAME VARCHAR2(127 CHAR),
   map member function f return varchar2
Type created.
CREATE OR REPLACE TYPE BODY TREE_NODE AS
  map member function f return varchar2 is
  begin
     return NODE_NAME;
  end f;
end;
Type body created.
CREATE OR REPLACE TYPE TREE_NODES AS TABLE OF TREE_NODE NOT NULL
Type created.
SQL> select  *
  2    from  test_coll
  3  /
NODE_ID NODE_VALUE NODE_LEVEL
1       A                   1
2       B                   2
3       C                   3
4       D                   4
5       E                   1
6       F                   2
7       G                   3
8       H                   4
8 rows selected.
SQL> Now:
SELECT CAST(COLLECT(REGION_NODES)     AS TREE_NODES) AS REGIONS,
       CAST(COLLECT(DEPARTMENT_NODES) AS TREE_NODES) AS DEPARTMENTS,
       CAST(COLLECT(AREA_NODES)       AS TREE_NODES) AS AREAS,
       CAST(COLLECT(CENTRE_NODES)     AS TREE_NODES) AS CENTRES
  FROM (
        SELECT  ROWID RID,
                NODE_LEVEL,
                TREE_NODE(NODE_ID, NODE_VALUE) AS NODE
          FROM  TEST_COLL
PIVOT (MAX(NODE) FOR NODE_LEVEL IN (
                                     1 AS REGION_NODES,
                                     2 AS DEPARTMENT_NODES,
                                     3 AS AREA_NODES,
                                     4 AS CENTRE_NODES
REGIONS(NODE_KEY, NODE_NAME)                         DEPARTMENTS(NODE_KEY, NODE_NAME)                     AREAS(NODE_KEY, NODE_NAME)                           CENTRES(NODE_KEY, NODE_NAME)
TREE_NODES(TREE_NODE('1', 'A'), TREE_NODE('5', 'E')) TREE_NODES(TREE_NODE('6', 'F'), TREE_NODE('2', 'B')) TREE_NODES(TREE_NODE('7', 'G'), TREE_NODE('3', 'C')) TREE_NODES(TREE_NODE('8', 'H'), TREE_NODE('4', 'D'))
SQL> SY.